Mapping cDNAs

The homology between YAC and cDNA library vectors need not be a
problem. The hybridizations can be carried out in the presence of
an excess of unlabeled cDNA vector, which should leave the plant
derived sequences as the only available sites for hybridization.
This is exactly analogous to the use of Kpn and Alu repeats in
hybridization of cosmid clones to human chromosomes - the cold
repeated DNAs block hybridization of the probe to repetitive
sequences and allow unambiguous localization of cosmids on
cytological spreads. The same trick should work on filter
hybridizations as well.
Ross Whetten